1. Field of the Invention
The present invention relates to image forming apparatuses, and in particular, relates to a technique for identifying patch images that are formed on a recording material for image correction.
2. Description of the Related Art
Improvement in image quality of images output by color image forming apparatuses such as color printers and color copiers has been sought. Density tones and stability thereof in output images are important elements that decide image quality, and thus it is necessary to suppress variation in density due to environmental changes or long-time use in color image forming apparatuses.
For this reason, Japanese Patent Laid-Open Nos. 2000-039747 and 2006-308812 each disclose a configuration in which toner images for detecting density or a color value (hereinafter referred to as “patch images”) are formed on a recording material, and the density or the color value of the patch images formed on the recording material are detected, thereby correcting the density or the color value of the toner images. Here, it is desirable to form a large number of patch images in order to improve correction accuracy, and in order to attain this, patch images formed at various densities or in various colors are arranged on a recording material while providing no interval therebetween.
At this time, since no interval is provided between patch images, in Japanese Patent Laid-Open No. 2000-039747, patch images are arranged such that a difference in density between patch images adjacent to each other is greater than or equal to a predetermined value, and the red, green and blue values are detected using an RGB color sensor, thereby identifying each patch image. Note that as a color sensor, a red LED, a green LED and a blue LED are used in Japanese Patent Laid-Open No. 2000-039747, and a combination of a white LED and RGB filters is used in Japanese Patent Laid-Open No. 2006-308812.
In order to realize better color reproducibility in a color image forming apparatus, it is desirable to detect patch images of higher-order colors such as secondary colors and tertiary colors (mixed-color patch images), in addition to single chromatic colors produced by cyan, magenta and yellow. However, when patch images of higher-order colors are considered, conventional techniques may have the problem described below. That is, if a plurality of mixed-color patch images are arranged while providing no interval therebetween, it may be impossible to detect the boundary between patch images adjacent to each other depending on the color relation between the patch images.
For example, it is assumed that two patch images 53 and 54 are adjacent to each other, and that the spectrum of light reflected by each patch image with respect to a white light source is as shown in FIG. 16. Note that in FIG. 16, reference numerals 50, 51 and 52 respectively indicate the spectral transmission curves of red, green and blue (RGB) filters. In the case of FIG. 16, light reflected by the patch images 53 and 54 take substantially the same RGB values. Accordingly, it is difficult to identify patch images adjacent to each other. In other words, identification accuracy is deteriorated.